The governing body of any municipality indebted on account of outstanding municipal bonds is authorized in its discretion to appoint a fiscal agent located in some city within or without the state, or if deemed convenient, one such agent, in each of two cities. Every such fiscal agent must be an incorporated bank or trust company authorized by the laws of the United States or of the state in which it is located to do a banking or trust company business. The custodian of the sinking funds of the municipality, when necessary, shall deposit with such fiscal agent such sums of money as are required for the payment of the principal or interest of municipal bonds.